A pressure sensing device for injection molding machines

Abstract: A hydraulic pressure cylinder incorporating a double-rod type piston 3 - for example injection cylinder 2, when the hydraulic pressures of fore and rear oil chambers are respectively detected by a pair of pressure sensors 4, 5; and subsequently obtaining detected pressure value Pd from the difference between respective oil chamber pressure measurements Pdf, Pdr based on the outputs respectively from pressure sensors 4, 5, the detected pressure value Pd of period Ts during which detected pressure value Pd becomes zero theoretically, is calculated in advance, then while storing calculated detected pressure value Pd as correction value A, at the time of pressure sensing, correcting the detected pressure value Pd according to correction value A, whereby the accurate zero adjustment point may always be maintained without being influenced by a temperature drift, enabling an accurate and high-precision pressure sensing.

Description: 1. Field of the Invention The present invention relates to a pressure sensing device for injection molding machines, to detect the pressure of a hydraulic cylinder such as an injection cylinder, etc. 2. Description of the Relevant Art An injection molding machine capable to move a screw by an injection cylinder incorporating a double-rod type piston is disclosed in Japanese Patent Application Publication Laid-Open No. 5 (1993)-50483. This type of injection cylinder is provided with fore and a rear rod, in front and behind the piston both rods being prepared in an identical diameter, whereby a fore oil chamber is provided in front of the piston within the injection cylinder while a rear oil chamber is likewise formed in the rear thereof. Therefore, control of the screw in its forward and backward movements, and stop (position) can be performed by simultaneously. applying hydraulic pressure to each of these oil chambers. It is noted here that the fore rod is coupled to the rear end of the screw inserted in a barrel cylinder while the rear rod is spline-joined to the shaft of an oil motor installed at the rear end of the injection cylinder. Normally, when detecting screw pressure hydraulic pressures of the oil chamber inside the injection cylinder is detected by using a pressure sensing device( see Japanese Granted Patent publication Laid-Open No. 3 (1991)-12528). Therefore, with the injection cylinder having a double-rod type piston built in, the hydraulic pressures of the fore and rear oil chamber are respectively detected, using a pair of pressure sensors, and after the detected pressure therefrom are respectively amplified by head amplifiers, the outputs therefrom are subsequently sent to a deviation computing unit wherein a pressure deviation across the fore and rear oil chambers is calculated; this deviation denotes a screw pressure to be detected. Meanwhile, such a pressure sensing device as described above uses a strain gauge as a pressure sensor, with the measurement of pressure provided thereby being inclusive of those errors stemming from the functional fluctuation characteristic of the strain gauge itself and positional irregularity of the mounting thereof to the injection cylinder. In view of this, therefore, the pressure deviation computing unit or head amplifier is normally installed with a zero adjusting and a gain adjusting circuit whereby zero point and gain adjustments may be implemented at the stage of initial adjustment. Regrettably, however, when a temperature drift occurs due to various factors such as the usage environment of an injection molding machine, time zone during which the machine is run and so forth, even if a zero point adjustment at the stage of initial adjustment is performed it may undergo some deviation from the adjusted point, resulting in a failure of undertaking accurate high-precision pressure detection and a deterioration of quality of molded products. Summary of the Invention According to the present invention there is provided a pressure sensing device for an injection molding machine, which device includes a pair of pressure sensors operable to detect the hydraulic pressures respectively of a fore and a rear oil chamber of a hydraulic cylinder incorporating a double-rod type piston, and a deviation computing unit operable to calculate a detected pressure value from the difference between respective oil chamber pressure measurements based on the outputs of said pressure sensors, characterised in that said pressure sensing device further comprises: a correction value computing unit operable to calculate a correction value; correction value memory operable to store a calculated correction value; and a pressure correcting unit operable to correct a detected pressure value, using said stored correction value, at the time of pressure sensing, and in that the pressure sensing device is operable to calculate a correction value in a given period subsequent to the completion of initialization with the power supply turned on, in which the detected pressure value becomes theoretically zero, and in a given period subsequent to the completion of an injection cycle by the injection molding machine, in which the detected pressure value becomes theoretically zero. Thus an embodiment of the present invention provides a pressure detecting device for injection molding machines, which is capable of undertaking high-precision pressure sensing via constant maintenance of an accurately adjusted zero point, without being influenced by a temperature drift, whereby the molding quality of product may be improved. In the present invention, with a view to achieve this object, when configuring a pressure detecting device provided with a hydraulic cylinder incorporating double-rod type piston 3, for example, a pair of pressure sensors 4 and 5 detecting the hydraulic pressures of fore and rear oil chambers 2f, 2r respectively of injection cylinder 2 and a deviation computing unit 5 which obtains detected pressure value Pd from the deviation between detected pressures oil chambers 6 Pdf and Pdr based upon outputs of each pressure sensor 4 and 5, the device should be especially configured to comprise a correction value computing unit 7 which obtains as correction value A, the pressure detected value Pd during the period where the pressure detected value Pd becomes theoretically zero, correction value memory 8 which stores calculated correction value A, and pressure correcting unit 9 correcting pressure detected value Pd with the correction value A. Thus, basically, the pressures of fore and rear oil chamber 2f, 2r of injection cylinder 2 are detected by pressure sensors 4 and 5, and from the respectively deviation between respective oil chamber internal pressure measurements Pdf, Pdr based on the outputs of pressure sensors 4 and 5, differential pressure computation unit 6 calculates the intended detected pressure value. During the run of an injection molding machine, there exists a period Ts during which said detected pressure Pd becomes zero theoretically. Since no pressure control is implemented during a prescribed period which follows the completion of initialization with the power supply turned on or a prescribed period subsequent to the completion of an injection process, each of these periods is the period Ts during which detected pressure value Pd becomes theoretically zero. Noting the existence of this period Ts, the pressure sensing device of the present invention automatically corrects the detected pressure value Pd at the time of pressure sensing, with the correction value A which is obtained during period Ts. Namely, as the detected pressure value Pd obtained from deviation computing device 6 during period Ts is a margin of error during period Ts where detected pressure value Pd becomes zero theoretically (fundamentally), correction value computing unit 7 computes beforehand pressure value Pd of that time as correction value A. Averaging detected pressure values Pd which have been obtained through plural times sampling when obtaining correction value A will lessen the margin of error of correction value A. Correction value A obtained in this way is stored in correction value memory 8. Meanwhile, during pressure sensing for pressure control, etc. detected pressure value Pd calculated by deviation computing unit 6 is corrected by pressure correcting unit 9 according to correction value A stored in correction value memory 8. Namely, the correction value A is added to or subtracted from calculated detected pressure value Pd, whereby accurate detected pressure value Pda is obtained as a result. (Brief Description of the Drawings) Fig.1 is a block diagram illustrating the injection molding machine including a pressure sensing device to which the present invention relates. Fig. 2 is a flowchart depicting the process of pressure sensing with the pressure sensing device of the present invention. Fig. 3 is a timing chart showing the profile of pressure changing to illustrate the pressure sensing process using the pressure sensing device to which the present invention relates. (Detailed Description of the Preferred Embodiments) The preferred embodiments of the present invention are illustrated hereunder in detail with reference to the accompanying drawings. First with reference to Fig. 1, the configuration of injection molding machine M installed with pressure sensing device 1 in one preferred embodiment of the present invention is illustrated. Injection molding machine M comprises injection unit 10 and die clamping unit 20. Injection unit 10 has barrel cylinder 11 with injection nozzle 12 mounted at the fore end thereof and hopper 13 at the rear end thereof, wherein said barrel cylinder has screw 14 incorporated therein, and screw drive 15 is coupled to the rear end of said barrel cylinder. Screw drive 15 includes injection cylinder (hydraulic cylinder) 2 with built-in double-rod type piston 3, wherein fore rod 3f of piston 3 is coupled to the rear end of screw 14 while rear rod 3r is spline-joined to the shaft of oil motor 16 arranged at the rear end of injection cylinder 2. Further, the diameters of fore and rear rod 3f, 3r are identical. Die clamping unit 20 illustrated in an imaginary line supports dies 21 and opens and closes the dies 21. In the meanwhile, hydraulic control circuit C is connected to injection cylinder 2. Hydraulic control circuit C includes hydraulic circuit 30 and control circuit U. Hydraulic circuit 30 is provided with 4-port servo valve 31, with A port thereof joined to fore oil chamber 2f of injection cylinder 2, B port to rear oil chamber 2r thereof, P port to source of hydraulic pressure (a hydraulic pump or an accumulator) 32, and T port to oil tank 33 respectively. Also the control circuit U is provided with pressure sensing device 1, of the present invention. Pressure sensing device 1 includes pressure sensor 4 with a strain gauge to detect the hydraulic pressure within fore oil chamber 2f of injection cylinder 2; pressure sensor 5 identical with pressure sensor 4 and which detects the hydraulic pressure in rear oil chamber 2r of injection cylinder 2; head amplifiers 34, 35, each including a bridge circuit connected to pressure sensors 4 and 5; and deviation computing unit 6 to calculate detected pressure value Pd, intended to be obtained, from the difference between oil chamber internal pressure measurements of Pdf and Pdr outputted from head amplifiers 34, 35, and the calculated detected pressure value Pd becomes the detected value of injection pressure or holding pressure. Pressure sensing device 1 according to the present invention further includes correction value computing unit 7 with a function to obtain as correction value A the detected pressure value Pd obtainable from deviation computing unit 6 during period of Ts over which detected pressure value Pd becomes theoretically zero; correction value memory 8 to store calculated correction value A; and pressure correcting unit 9 which corrects detected pressure value Pd with said correction value A at the time of pressure sensing, namely, by adding said correction value to or subtracting from the detected pressure value Pd. Meanwhile, in control circuit U, 36 refers to deviation detector to obtain the difference between detected pressure value Pda which is obtained from pressure correcting unit 9 and previously set pressure Ps, 37 refers to pressure compensating unit to obtain a pressure manipulated variable being pressure compensated against the deviation confirmed by deviation detector 36, 38 to position sensor to detect the position of screw 14, 39 to speed converting unit for obtaining a screw speed from the screw position detected by position sensor 38, 40 to deviation detecting unit to obtain the deviation between a detected speed provided by speed converting unit 39 and a set speed, 41 to speed compensating unit to provide a speed manipulated variable being speed compensated, 42, 43 to respective switching function unit selecting pressure and speed manipulated variables, 44 to servo controller for converting a pressure or a speed manipulated variable into a control signal thereby to regulate servo valve 31, and 45 to a control unit undertaking various controls (servo sequence control, machine control, etc.). Next, referring to Fig. 1, Fig. 2, and a flowchart in Fig. 2, a description is made hereunder about a part of the controls concerned with the run of injection molding machine M, including the performance (pressure sensing method) of pressure sensing device 1 in the preferred embodiment of the present invention. Basically with pressure sensing device 1, the pressures of fore and rear oil chamber 2f, 2r of injection cylinder 2 are detected by pressure sensors 4, 5, and after the measurements of these pressures are amplified by head amplifiers 34, 35, they are transferred subsequently to deviation computing unit 6, whereby pressure value Pd which is the difference between oil chamber internal pressures Pdf and Pdr outputted from head amplifiers 34 and 35, is obtained. Incidentally a given period subsequent to the completion of an injection process becomes period Ts, during which the difference between the respective measurements of fore and rear oil chamber internal pressures based on the outputs becomes theoretically zero for no pressure control is undertaken during this period, of pressure sensors 4, 5. Namely, as shown in Fig. 3, the injection process comprises a speed control zone within which an injection and a fill-up step is effected, and a pressure control zone wherein pressure holding is carried out, and the so-called relief of pressure is performed, following the ending of pressure control zone at point Se, whereby rear oil chamber 2r of injection cylinder 2 opens to oil tank 33 (steps 50 through 52). Control unit 45 monitors the relief of pressure under way, and when point Sp specified in Fig. 3 where the pressure relief ends, is reached, correction value computing unit 7 calculates correction value A. To be more particular, correction value computing unit 7 with its timer function counts up (step 53) given time interval Ti, followed by sampling to detected pressure value Pd obtained by deviation computing unit 6 (steps 54, 55) after time is up. In this case the sampling is done "n" times according to a certain cycle. Further, in view of the fact that the detected pressure value Pd undergoing sampling is the detected pressure value Pd of period Ts during which it becomes theoretically zero, it is considered as an error of pressure produced in consequence of a pressure drift. Meanwhile, when the number of sampling reaches "n" times (step 56), correction value computing unit 7 calculates (step 57) the mean value of each sampled detected pressure value Pd, and the calculated mean value is subsequently stored (step 58) as correction value A in correction value memory 8. On the other hand, in the next injection process wherein pressure control is performed, detected pressure value Pd is subjected to correction with the stored correction value A. In this case, since feed back control of pressure is performed during control of pressure, first the pressure sensing device 1 detects the detected pressure value Pd. Namely, the respective pressures of fore and rear oil chamber 2f, 2r of injection cylinder 2 are detected by sensors 4, 5, and subsequently, deviation computing unit 6 calculates the detected pressure value Pd from the difference between detected fore and rear oil chamber internal pressure Pdf and Pdr based on the outputs of pressure sensors 4, 5. The information of both calculated detected pressure value Pd, and correction value A stored in correction value memory 8 is transferred to pressure correcting unit 9 wherein calculated detected pressure value Pd undergoes correction (step 59) by the correction value A. More precisely, the correction is effected with correction value A either added to or subtracted from calculated detected pressure value Pd, whereby corrected accurate detected pressure value Pda may be obtained. Further, the detected pressure value Pda after being corrected and obtained from pressure sensing device 1 is transferred to deviation detector 36 wherein the deviation between said detected pressure value Pda and previously set pressure setting value Ps is obtained. This deviation of pressure goes through pressure compensation by pressure compensating unit 37, and transferred to servo controller 44 as a pressure manipulated variable. As a result a control signal from servo controller 44, is given to servo valve 31, to control hydraulic pressure while pressure regulated oil is supplied to fore and rear oil chamber 2f, 2r of injection cylinder 2, and screw 14 is pressed thereby. On the other hand, since detected pressure value Pda is available from pressure sensing device 1, the pressure feedback system effects pressure control in a manner that detected pressure value Pda matches pressure setting value Ps. In this case, switching function unit 43 is turned OFF and switching function unit 42 is turned ON by control unit 45. Incidentally, for pressure sensors 4, 5, an allowable range of pressure drift (approximately 10% their pressure rating) is prescribed. Therefore, should each of these pressure sensors be found to have gone through zero correction beyond prescribed value, these pressure sensors may be considered to be defective or malfunctioning. Therefore, in case of such trouble, actuating an alarm or suspending the control thus far in progress must be done. One preferred embodiment of the present invention has been described in the foregoing, but the present invention is by no means limited to such an embodiment referred to above. For example a case of configuring the above preferred embodiment with hardware has been explained. However, the same process may be performed with software. The case of detecting pressure of piston in injection cylinder has been illustrated, however, the subject pressure sensing device of the present invention is likewise applicable to another hydraulic cylinder such as a die clamping cylinder, etc. Furthermore, in said embodiment, a given period subsequent to the completion of the injection cycle is exemplified as a period during which the differential pressure becomes theoretically zero, together with a given period after the completion of initialization with the power supply turned. Other, similar periods may also be selected. Particularly, the fact that the detected pressure value theoretically becomes zero includes not only the instance where each of the pressure measurements of fore and rear oil chambers are zero but also another instance where the respective pressure measurements of these oil chambers is given with an identical pressure as when screw position control is carried out. With the pressure sensing device of the present invention it is allowed that the frequency of calculating correction value and the number of samplings or the timing, etc. (interval of correction) correction with corrected value may be selected optionally. Further, the subject pressure sensing device may have its configurational particulars, processing method, and so forth modified according to the appended claims.

Claims: A pressure sensing device (1, U) for an injection molding machine (M), which device (1, U) includes a pair of pressure sensors (4, 5) operable to detect the hydraulic pressures respectively of a fore (2f) and a rear (2r) oil chamber of a hydraulic cylinder (2) incorporating a double-rod type piston (3), and a deviation computing unit (6) operable to calculate a detected pressure value (Pd) from the difference between respective oil chamber (2f, 2r) pressure measurements (Pdf, Pdr) based on the outputs of said pressure sensors (4, 5), characterised in that said pressure sensing device further comprises: a correction value computing unit (7) operable to calculate a correction value (A); correction value memory (8) operable to store a calculated correction value (A); and a pressure correcting unit (9) operable to correct a detected pressure value, using said stored correction value (A), at the time of pressure sensing, and in that the pressure sensing device is operated to calculate a correction value (A) in a given period subsequent to the completion of initialization with the power supply turned on, in which the detected pressure value becomes theoretically zero, and in a given period subsequent to the completion of an injection cycle by the injection molding machine, in which the detected pressure value becomes theoretically zero. A pressure sensing device as claimed in claim 1, wherein said correction value computing unit (7) calculates a correction value (A) by averaging detected pressure values obtained in a plurality of samplings. A pressure sensing device as claimed in claim 1 or 2, wherein error processing means is provided in case the correction value (A) exceeds a prescribed value. A pressure sensing device as claimed in claim 1, 2 or 3, wherein the hydraulic cylinder is an injection cylinder (2).

Inventors: ARAI TSUYOSHI, ARAI, TSUYOSHI, ARAI, TSUYOSHI, NISSEI PLASTIC IND.CO.,LTD.
Applicants: NISSEI PLASTICS IND CO, NISSEI PLASTIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: B29C45
IPC codes: B29C45